Units of energy: What is a joule?

The joule or joule is a basic unit of the International System of Units that is used to measure work, energy and heat. It is generally symbolized by the letter « J » and is related to other units, such as the newton, the watt, the calories and the electron volt.

Joule unit (J): definition

This unit owes its name to James Prescott Joule, an English physicist who made various contributions to science in the 19th century. Among his investigations, his works on thermodynamics, electricity and energy stood out. 

The measure joule or joule ( J ) can be explained as the amount of work that a constant force of one Newton does along one meter , in the same direction. Therefore, one joule is equal to:

1J = 1N. m = (Kg . m/s ² ) . m = (kg . m ² ) / s ²

In addition, the joule can be defined as the work required to produce one watt or power for one second (Ws) . Or as the work needed to move a charge of one coulomb through a voltage of one volt (VC) .

Unit J equivalences with other units

To better understand what unit J consists of, its equivalences with other units can be taken into account. For example, 1 J is equal to:

• 0.238902957 cal (calories)
• 1 W s (watt second)
• 6.2415 × 1018 eV (electronvolts)
• 1 C V (coulomb per volt)
• 1 N m (newton per meter)
• 0.00987 atm L (atmosphere per liter)
• 1 Pa m³ (pascal per cubic meter)

J unit uses and examples

The joule is commonly used in Physics and Chemistry to measure the energy and heat of a body. For example, in everyday life, one joule is equal to:

• the kinetic energy of a tennis ball moving 6 meters per second;
• the amount of energy needed to throw a medium tomato one meter,
• or the release of energy produced by bursting the same tomato at a height of one meter;
• the amount of electricity required to light a 1 W LED lamp for one second;
• The energy released by a person at rest for one hundredth of a second.

Another case where the unit joule is used is in the formula  Q = P xt , where “Q” is the energy or heat generated (which is also represented by the letter E and can be measured in joules or calories), “P” the power consumed (measured in watts) and “t” the elapsed time (measured in seconds). The energy is generated because all systems tend, by nature, to seek a state of rest. Therefore, the movement of its particles will be as little as possible. Since, even in a state of rest, systems exchange energy with their surroundings, the joule can be used to measure it. Because it is a very small unit, multiples of the unit joule are commonly used, such as:

• kilojoule (kJ): 10 ³ J
• megajoule (MJ): 10 ⁶ J
• gigajoule (GJ): 10 ⁹ J
• terajoule (TJ): 10 ¹² J

Bibliography

• Atkins. physical chemistry . 2020 (9th Edition). Pan American.
• Petrucci, R.H. General Chemistry, Principles and Modern Applications . 2017 (11th Edition). Spain. pearon.
• Lleó, A. & Lleó, L. Great manual of physical magnitudes and their units. 2008. Spain. Editorial Diaz de Santos.